EXPERIMENTAL
51, 337-346
NEUROLOGY
Reflex
Figures JANETT
Departwent
(1976)
in the Decerebrate TRUBATCH
of Physiology,
New
A.
AND
York
and Spinal
VAN
Medical
HARREVELD
College,
Valhalla,
State
1
New
York
10595
aud Biology
Division,
California
Imtitute Received
of Technology, October
Pasadetta,
California
91125
21,1975
The flexors and extensors of the hind limb of the cat were simultaneously activated by stretch of a homolateral flexor muscle or by a nocuous stimulus to the homolateral paw under the conditions of the present experiments, in which the muscles under investigation were maximally retracted after detachment of the tendons from their insertions. The simultaneous activation of flexors and extensors was observed in acute and chronic spinal preparations, and sometimes in decerebrate animals. However, when in such preparations the extensor muscle was made to contract by loading its tendon, stretch of the antagonistic flexor muscles and pinching of the toes of the homolateral leg usually resulted in extensor inhibition and flexor excitation, in agreement with the classical tenet of reciprocal innervation. The reflex figure seems to be to an important extent the result of the inflow of afferent impulses.
INTRODUCTION A striking feature of the functional organization of the spinal cord is the way in which the excitatory and inhibitory effects of a stimulus tend to influence motoneurons reciprocally, causing contraction of certain muscles and inhibition of their antagonists. Thus, for example, Liddell and Sherrington (18) demonstrated in decerebrate preparations that the stretch of a muscle reflexly excited its own motoneurons, while inhibiting those of antagonistic muscles. Reciprocal innervation has been documented in spinal preparations as well ( 1). The reflexes in preparations which had recovered from asphyxiation of the spinal cord for about 3.5 min, in general did not exhibit the reciprocal principle (21). Especially during “late tone,” the stretch of flexor muscles elicited contractions in both flexors and extensors, as did nocuous stimuli. 1 This investigation Public Health Service.
was
supported
in
part
337 Copyright All rights
0 1976 by Academic Press, Inc. of reproduction in any form reserved.
by
Grant
NS-09493
from
the
U.
S.
338
TRUBATCH
AND
VAN
HARREVELD
Also the final phase of “secondary tone” was characterized by the absence of reciprocal innervation. In the present paper preparations and conditions are described in which, even without asphyxiation, spinal reflexes did not show a reciprocal distribution. METHODS Chronic and acute spinal cats were prepared respectively 2 to 3 weeks or 1 day before the terminal experiment. Under pentobarbital narcosis the dura was ligated at T 10-11, severing the spinal cord. In the terminal experiment the trachea of normal and spinal cats was cannulated under ether narcosis. The carotid arteries were ligated and one was cannulated for blood pressure recording. The preparation was decerebrated by cutting the brain ,stem, using the bony tentorium as a guide. Most of the forebrain was removed and the narcosis discontinued. The tendons of the quadriceps, the gastrocnemius-soleus, the semitendinosus, and the anterior tibia1 muscles on the homolateral and contralateral side were isolated and freed from the surrounding tissue. A strong cotton thread was attached to each of them. The condyles of the femurs and the distal end of the tibias were fixed on a vertical board with screws. The threads attached to the tendons were led over pulleys and loaded with 5-g weights to keep the threads under slight tension. Larger weights could be attached to the small weights to stimulate the stretch receptors in the muscles. Electromyograms were led off from the quadriceps and gastrocnemius-soleus muscles and in some experiments from the tibia1 muscle with electrodes placed in the muscle belly. The potentials, after amplification, were recorded on one or more channels of a Grass polygraph or a Physiograph, and in many experiments integrated by another channel. The blood pressure was monitored continually. RESULTS Preparations that had been operated the day before the terminal experiment showed an unexpected distribution of reflex activity. Table 1A shows four typical experiments. In all preparations the extensor muscles (quadriceps and gastrocnemius-soleus, q and g) were activated by stretch of their own tendons as well as of their synergist (gastrocnemius-soleus or quadriceps, g or q) . However, in contrast to the results expected from the principle of reciprocal innervation, stretching the antagonist flexors (semitendinosus and anterior tibial, s and t) of the homolateral leg also led in most instances to excitation of the extensors. Extensor activity was observed in three of the preparations (Table lA, 1, 2 and 3) after loading some of the muscles of the contralateral leg (quadriceps, semitendinosus,cq, CS) . Acute
Spinal
Preparations.
REFLEXFIGURES
339
Pinching of the foot, which elicited a marked contraction of the flexor muscles (flexion reflex), was expected to cause inhibition of the homolateral extensors. In most preparations, however, the extensor muscles contracted strongly on pinching the homolateral paw (Table 1A). Pinching the contralateral paw had in general no effect but caused in two preparations (Table lA, 3, 4) a slight contraction or inhibition of the extensors. Decerebrate Preparations. Decerebrate rigidity as evidenced by a pronounced extensor contraction, particularly in the front legs, appeared in most animals soon after the ether narcosis was discontinued. The hind legs, which usually showed less extensor rigidity, always exhibited brisk knee and ankle reflexes. In all preparations loading the tendons of the quadriceps, gastrocnemius, and tibia1 muscleswith weights of 200 or 500 g resulted in an enhancement of their electromyogram. These preparations often showed a significant reflex response only in the stretched muscle (Table lB, Fig. lA-J), although in some experiments contractions were elicited by synergists (Table IB, 7, 8). In preparations with a marked spontaneous background activity, or during activation of the extensor muscle by stretch, the contraction was often inhibited by simultaneous stretch of the flexors (Table lB, Fig. IK). However, in one preparation (Table lB, 7) stretch of the homolateral (and contralateral) semitendinocus muscles (s and csj caused contraction of extensors. TABLE
1 cs
A : Acute
1 2. 3 4
cg
ct
0 i 00 i
0 i0 0 0 i
P
CD
Spinal
Quad Gast Quad Gast Quad Gast Quad Gast
y’ 0” V) 0 0
0
+++++ A++ +++ +:+
0” 0 0 T -
B : Decerebrate i 7 8
Quad Quad Cast Quad Gast Quad Gast Tibia1
+++ +“+ +I+ 0
-
9
11
Quad Gast Quad Gast Quad Gast
+++ ++ O/$7
++
+++
33
+
+0+ ++ C : Chronic
10
0 0 0
++++++J+ ‘-;-“ +‘+’ +‘+ +‘+
i Spinal
+Y-
i!
++ +’
+I/-
s 0 0
“+’ 0
A- iI‘:T 0 0 0 0
+++ ++‘i‘+‘$++
a q-quadriceps. s-semitendinosus. g-gastrocnemius, t-anterior tibial, c-contralateral. p-pinching. B Strength oi contraction is roughly indicated by one to three + signs. inhibition by a - sign. effect by 0. Change of contraction to inhibition by stretch of the recording muscle is indicated notation +/ -.
++ :: ‘i++ and no by the
340
TRUBATCH
a
.4ND
VAI’i
H.4RREVZLD
cs- tt- w-
FIG. 1. The response of the gastrocnemius-soleus (g) muscle to stretch of the quadriceps (q), semitendinosus (s), gastrocnemius-soleus (g) and tibia1 (t) muscles and to pinching the paw (p) of the homolateral leg. Stretch of the muscles of the contralateral side is indicated by cq, cs, cg and ct, and contralateral pinch by cp. The bottom record is the electromyogram and the top is its integral. Stretch or pinch was applied during the time indicated by the central line. All muscles were loaded with 500-g weights. Records A-M show that in this decerebrate preparation the gastrocnemius muscle responded only to loading of its own tendon. In records K-M, the gastrocnemius was first loaded with a 500-g weight. Subsequent loading of the homoiateral anterior tibia1 muscle caused a slight inhibition of the myogram of the stretched gastrocnemius muscle ; homolateral pinching inhibited the activity in this muscle completely, heterolateral pinching caused a slight increase. The animal’s spinal cord was then transected and records N-V show the altered response of this same muscle 2 hr later. The vertical scale indicates 150 mV for the myogram; the horizontal marker, in this and all other records, is 10 sec.
In all these preparations, pinching of the paw elicited a marked contraction of the homolateral flexor muscles as expected from the flexion reflex. In some of the experiments (Fig. lL, M) this caused inhibition of homolateral and excitation of the contralateral extensor muscles (crossed extension). However, in other experiments (Table lB, 7, 8) the nocuous stimulus elicited a contraction in the homolateral extensors. In some experiments this excitation was changed to inhibition when the muscle was previously activated by stretch (Table lB, 7, 8). Figures 2A and B show an example of such an experiment. In this case the electromyogram was recorded simultaneously from the quadriceps, gastrocnemius, and tibia1 muscles. Pinching the homolateral paw (Fig. 2A) led to a pronounced activation of the tibia1 muscle and simultaneously a small contraction of the antagonist extensors (quadriceps, gastrocnemius) . Pinching the contra-
REFLEX
FIGURES
341
FIG. 2. Simultaneous recording of the electromyogram from the quadriceps, gastrocnemius, and tibia1 muscles. Records A and B are from a decerebrate preparation. In A the unloaded extensor muscles show a slight activation during both homolateral (p) and contralateral (cp) pinching of the paw. When the gastrocnemius was previously loaded with a 500-g weight (B), pinching resulted in a marked inhibition of the gastrocnemius myogram. This animal’s spinal cord was then transected and records C and D show similar responses 3 hours later. The vertical scale is 100 mV.
lateral foot caused activation of the quadriceps (as in crossed extension, but in addition there was a small effect on the tibia1 muscle. After the gastrocnemius was loaded with a 500-g weight (Fig. 2B), causing a marked increase of the myogram, pinching the homolateral foot caused a pronounced inhibition of the gastrocnemius activity and simultaneous tibia1 activation (as in reciprocal inhibiton). The reflex response of the decerebrate preparations to loading of muscles and to nocuous stimuli was in general in agreement with the tenet of reciprocal innervation, although exceptions were found. In several of the decerebrate preparations the spinal cord was transected subsequently and the experiment was repeated a few hours later. Figure lN-VL, which is a record of the same preparation as in Fig. lA-M in the decerebrate state, shows the effect of spinal transection. Loading the recording muscle (gastrocnemius, g) had now only a small effect but stretch of the flexor muscles (anterior tibia1 and semitendinosus, t and s) caused large extensor contractions. Also pinching the homolateral paw which caused in the decerebrate state complete inhibition of the extensor muscle now caused a marked contraction. In some instances it was possible to turn the contraction of extensors elicited by homolateral nocuous stimuli into inhibition by loading the extensor muscle. This was successful only when stretch of the extensor caused a marked excitation which was rarely observed in such preparations. Figure 2C. D is an example of such an observation. This preparation yielded Fig. 2A. B in the decerebrate state. Pinching the homolateral foot caused a simultaneous marked activation of both gastrocnemius and tibia1 muscles, and a smaller response of the quadriceps muscle (Fig. 2C). The response in the gastrocnemius was far more
342
TRUBATCH
AND
VAN
HARREVELD
marked than it had been under the same circumstances in the decerebrate state. However, when the gastrocnemius was activated by stretch (Fig. ZD), homolateral pinch (p) produced flexor excitation and extensor inhibition as expected from the principle of reciprocal activation. Chronic Spinal Preparations. In chronic spinal preparations the reflex excitability increases with time and after 2 to 3 weeks the preparations may respond to a relatively minor stimulus applied to the skin with extensive movements of the hind legs of considerable duration. Table 1C and Fig. 3 show effects of muscle loading in some representative preparations. In addition to a considerable effect of loading the recording quadriceps or gastrocnemius muscles, the myograms of these muscles were also enhanced by loading of several of the other muscles both homolaterally and contralaterally. Sometimes the effect of loading an antagonist was greater than stretching the recording extensor muscle (Figs. 3A and B). Only rarely did stretch of a homolateral flexor muscle inhibit the spontaneous electromyogram of the unloaded recording extensor muscle (Table lC, 11). However, in several cases excitation of the recording quadriceps or gastrocnemius muscle by one of its antagonists was changed to inhibition when the recording muscle was previously loaded (Table 1C). In the experiment shown in Fig. 4A, B, stretch of the homolateral semitendinosus and anterior tibia1 muscles enhanced the electromyogram of the unloaded recording quadriceps muscle. After loading the recording muscle with a 500-g weight, stretch of the semitendinosus muscle caused a decrease of the electrical activity in the muscle (Fig. 4D). The enhanced activity when the load was removed from the muscle demonstrates that this is indeed due to inhibition and not to fading of the stretch reflex. Stimulation of the anterior tibia1 muscle had a biphasic effect, first enhancing, then depressing the electromyogram of the recording muscle (Fig. 4E). Again the enhanced activity after removal of the load on the anterior tibia1 muscle proved the latter to be due to inhibition. Homolateral and contralateral pinching of the paw led in general to an increase in the electrical activity of the unloaded extensor muscle (Table
FIG. 3. The responses of the quadriceps muscle of a chronic spinal preparation to stretch of four homolateral and four contralateral leg muscles, and to pinching of the paws. For an explanation of the records see Fig. 1. The muscles on the homolateral side were stimulated with 200-g weights; on the contralateral side 500 g was used. The vertical scale is 300 mV for the myogram.
REFLEX
FIGURES
343
FIG. 4. The change of excitation to inhibition in a chronic spinal preparation. For an explanation of the records see Fig. 1. A, B, and C show the response of the gastrocnemius-soleus muscle to stretch of the semitendinosus and tibia1 muscles with 500-g weights and to homolateral pinching. In D, E, and F, a 500-g weight was attached to the recording gastrocnemius-soleus muscle a few seconds before stretch of the flexor muscles and of homolateral pinching. The vertical bar indicates 200 mV for the myogram.
1C and Fig. 31, J). The activation by homolateral pinching could in several experiments be changed to inhibition by loading the recording muscle. Figure 4C shows the enhancement of the electromyogram of the unloaded quadriceps muscle by pinching the homolateral paw; Fig. 4F shows the effect of the same stimulus on the loaded recording muscle which is a marked inhibition. DISCUSSION Stretch of a muscle elicits impulses in the primary (1A) and secondary (II) endings of the muscle spindles and in the 1B fibers from the Golgi tendon organs. Activation of the 1A system leads to excitation of the motoneurons of homonymous and synergist muscles and inhibition of antagonistic motoneurons. The tendon organs inhibit the motoneurons of the stretched muscle (autogenic inhibition) and reflexly excite antagonist motoneurons. A third group of thinner afferent fibers (group II), originating mainly in the secondary muscle spindle endings, usually cause a weak excitation of flexor motoneurons and inhibition of extensors (2, 3, 7, 17) although other effects have been observed (7. 11, 23 j. The widespread presynaptic inhibition of primary afferents by muscle, tendon, joint, and skin afferents further complicates this picture. In addition, the discharge of the motoneurons which innervate the intrafusal fibers of the muscle spindles is affected by stretch of the muscle (4, 10, 12, 20) and in this way indirectly modifies the discharge of the spindle afferents. A stimulus such as the stretch of a muscle therefore has excitatory as well as an inhibitory effect on its own motoneurons (9, 13, 19). Furthermore, such stimuli have effects not only on motoneurons of the stretched muscle, but on homolateral as well as contralateral synergists and antagonists. In all these reactions to stretch, except for the activation of the homonymous motoneurons by the IA fibers, interneurons are involved.
344
TRUBATCH
AND
VAN
HARREVELD
Other peripheral receptors, in addition to the muscle stretch receptors, contribute to the input of the motoneurons. There are pressure receptors (Pacini’s corpuscles) and less well defined pain receptors in the muscle. The latter form a group of (thin) afferent fibers (group III) which elicit mainly flexion reflexes when activated. In the preparations used, larger and smaller cutaneous nerves were unavoidably cut. The injury to those nerves produces only a short-lasting discharge, after which they remain silent (22). In addition to this complicated peripheral innervation, the motoneurons are affected by descending activity from supraspinal centers. Especially the descending activity during decerebrate rigidity is of interest, Descending impulses excite the y motoneurons, causing the contraction of intrafusal fibers and an enhanced discharge of the primary sensory endings of the muscle spindles resulting in the reflex contraction of LYmuscle fibers, an activity known as the y loop (8, 15). Furthermore, stimulation of group II and III fibers elicits little or no reflex discharge of flexor motoneurons in decerebrate preparations, whereas after transection of the spinal cord impulses in these fibers cause large reflex responses, suggesting that during decerebrate rigidity descending impulses inhibit interneurons involved in these reflex activities (6, 16). The multiplicity of impulses impinging upon the motoneurons, both from the periphery and (in normal and decerebrate preparations) from higher centers determine the reflex figure in a given preparation under given circumstances. Reciprocal innervation is the reflex figure which was usually seen in decerebrate and spinal preparations. Other responses are possible, however. During voluntary movements both extensors and flexors are activated. The reflex figure can furthermore be changed markedly, not only by a change of the supraspinal input, as by spinal cord transection of a decerebrate preparation, but also by modification of the input from the periphery. Job (14) f ound that the enhancement of monosynaptic reflex responses during stretch of the gastrocnemius muscle in spinal preparations turned into a decrease when the muscle was cooled or its circulation impaired. Peripheral modification or reflex activity was also demonstrated by Clare and Landau (5), who showed that the response of the extensor digitorum brevis muscle to shocks applied to the tibia1 nerve was enhanced after crushing the innervation of the interosseus muscles. Also the present investigation provides an example of the modification of the reflex activity by changes in the afferent inflow. Stretch of a homolateral flexor muscle or homolateral nocuous stimuli caused in spinal preparations either contraction or inhibition of an extensor muscle depending on the state of retraction or stretch of the latter. When the tendon, which was detached from its insertion, was not loaded the muscle retracted, its length becoming even shorter than during maximal extension of the
REFLEX
FIGURES
345
joints in the normal state. Under these circumstances the afferent outflow from the muscle will be at a minimum, which is borne out by the observation that in the decerebrate state the spontaneous electromyogram is small (Fig. 2A, B) or absent (Fig. IA-J). Such retracted extensor muscles tended to be activated in spinal preparations by homolateral flexor stretch and pinching of the paw. Loading the tendon of the recording extensor muscle with a 200- to 500-g weight causes an outflow of impulses in 1A fibers which activate the muscle reflexly and in 1B and group II fibers which both inhibit the extensor motoneurons. The result of homolateral flexor stretch and nocuous stimulation was then often an inhibition of the contraction caused by activation of the 1A fibers. The reversal of the reflex effect of homolateral flexor stretch and nocuous stimulation thus seems to be caused by a change in the afferent inflow from the recording extensor muscle. In the retracted state in which the afferent outflow from the unloaded extensor muscle is at a minimum, the excitatory components of homolateral flexor stretch and pinching of the paw apparently override the inhibitory effect of these stimuli, causing the extensor contractions elicited consistently by these stimuli in acute and chronic spinal preparations (Table 1A and C). In the loaded state of the recording extensor muscle the combined inhibitory effect of 1B and group II stimulation, with the inhibitory component of the reflex activity caused by homolateral flexor stretch and pinch of the paw, seems to be able to overcome the excitatory component of the latter stimuli and the effect of 1A stimulation. In decerebrate preparations the extensor muscle usually exhibited reciprocal innervation. Even in the retracted state, homolateral flexor stretch or nocuous stimulation caused extensor contraction only occasionally (Table lB, Fig. 2A, B) The supraspinal inflow in these preparations which has excitatory as well as inhibitory components (see above) apparently tends to tip the balance of excitatory and inhibitory impulses impinging on the extensor motor neurons usually to the inhibitory side. The conclusion can be drawn from these experiments that reciprocal innervation does not depend on fixed neural connections in the spinal cord but is the result of the ratio of excitatory and inhibitory impulses reaching the motoneurons. A ratio favoring reciprocal innervation seems usually to have been present under the conditions of the experiments on reflex figures reported in the past. REFERENCES 1. BALLIF, L., J. F. FULTOK, decerebrate knee-jerks, break-shocks. Proc. R. 2. BIANCONI, R., R. GRANT,
and E. G. I. LIDDELL. 192.5. Observations on spinal and with special reference to their inhibition by single Sot. Series B, 98: 589-607. and D. J. REIS. 1964. The effects of extensor muscle
346
3.
4.
5. 6, 7. 8. 9. 10. 11.
12. 13. 14. 15.
16.
17.
18. 19.
20. 21. 22. 23.
TRUBATCH
AND
VAN
HARREVELD
spindles and tendon organs on homonymous motoneurones in relation to -y-bias and curarization. Acta. Physiol. Stand. 61 : 331-347. BIANCONI, R., R. GRANIT, and D. J. REIS. 1964. The effects of flexor muscle spindles and tendon organs of homonymous motoneurones in relation to y-bias and curarization. Acta. Physiol. Stand. 61: 348-356. BROWN, M. C., D. G. LAWRENCE, and P. B. C. MATTHEWS. 1968. Reflex inhibition by Ia afferent input of spontaneously discharging motoneurones in the decerebrate cat. 1. Physiol. (London) 198: S-7P. CLARE, M. H., and W. M. LANDAU. 1975. Distal hind-limb reflex responses in cats with largely intact spinal reflex circuits. Exp. Newel. 46: 47C-495. ECCLES, R. M., and A. LUNDBERG. 1959. Supraspinal control of interneurones mediating spinal reflexes. J. Physiol. (London) 147: 565-584. ECCLES, R. M., and A. LUNDBERG. 1959. Synaptic actions in motoneurones by afferents which may evoke the flexion reflex. Arch. Ital. Biol. 97 : 199-221. GRANIT, R., and B. R. KAADA. 1952. Influence of stimulation of central nervous structures on muscle spindles in cat. Acta. Physiol. Stand. 27: 130-160. GRANIT, R., J.-O. KELLERTH, and T. D. WILLIAMS. 1964. Intracellular aspects of stimulating motoneurones by muscle stretch. J. Physiol. (London) 174 : 435-452. GRILLNER, S. 1969. Supraspinal and segmental control of static and dynamic ymotoneurones in the cat. Acta. Physiol. Stand. 76: suppl. 327. HOLMQVIST, B., and A. LUNDBERG. 1961. Differential supraspinal control of synaptic actions evoked by volleys in the flexion reflex afferents in alpha motoneurones. Acta. Phq’sioE. Stand. 54: suppl. 186. HUNT, C. C. 1951. The reflex activity of mammalian small-nerve fibers. I. Physiol. (London) 115 : 456-469. HUNT, C. C. 1952. The effect of stretch receptors from muscle on the discharge of motoneurones. I. Physiol. (London) 117 : 359-379. JOB, C. 1953. Uber autogene Inhibition und Reflexumkehr bei spinalisierten und decerebrierten Katzen. Pfliigers Arch. Ges. Physiol. 25’6: 406-418. KOBAYASHI, Y., K. OSHIMA, and I. TASAKI. 1952. Analysis of afferent and efferent systems in the muscle nerve of the toad and cat. J. Physiol. (London) 117: 152-171. KUNO, M., and E. R. PERL. 1960. Alteration of spinal reflexes by interaction with suprasegmental and dorsal root activity. J. Physiol. (London) 151: 103-122. LAPORTE, Y., and P. BESSOU. 1959. Modification d’excitabilitb de motoneurones homonymes provoqueCs par l’activation des fibres affCrentes d’origine musculaire du groupe II. J. Physiol. (Paris) 51: 897-908. LIDDELL, E. G. T., and C. SHERRINGTON. 1925. Further observations on myotatic reflexes. Proc. R. Sot. Series B, 97 : 267-283. MCINTYRE, A. K. Central actions of impulses in muscle afferent fibers. In “Muscle Receptors,” Handbook of Sensory Physiology. Vol. III/Z, C. C. Hunt [Ed.], pp. 235-288. PROSKE, U., and D. M. LEWIS. 1972. The effects of muscle stretch and vibration on fusimotor activity in the lightly anesthetized cat. Brain Res. 46: 5549. VAN HARREYELD, A., and J. TRUBATCH. 1974. Reflex figures during asphyxial rigidity. Exp. Nmrol, 45 : 161-173. WALL, P. D., S. WAXMAN, and A. I. BASBAUM. 1974. Ongoing activity in peripheral nerve : Injury discharge. Exp. Nealrol. 45 : 576-589. WILSON, V. J., and M. KATO. 1965. Excitation of extensor motoneurons by group II afferent fibers in ipsilateral muscle nerves. J. Newophysiol. 28: 545-554.
51, 337-346
NEUROLOGY
Reflex
Figures JANETT
Departwent
(1976)
in the Decerebrate TRUBATCH
of Physiology,
New
A.
AND
York
and Spinal
VAN
Medical
HARREVELD
College,
Valhalla,
State
1
New
York
10595
aud Biology
Division,
California
Imtitute Received
of Technology, October
Pasadetta,
California
91125
21,1975
The flexors and extensors of the hind limb of the cat were simultaneously activated by stretch of a homolateral flexor muscle or by a nocuous stimulus to the homolateral paw under the conditions of the present experiments, in which the muscles under investigation were maximally retracted after detachment of the tendons from their insertions. The simultaneous activation of flexors and extensors was observed in acute and chronic spinal preparations, and sometimes in decerebrate animals. However, when in such preparations the extensor muscle was made to contract by loading its tendon, stretch of the antagonistic flexor muscles and pinching of the toes of the homolateral leg usually resulted in extensor inhibition and flexor excitation, in agreement with the classical tenet of reciprocal innervation. The reflex figure seems to be to an important extent the result of the inflow of afferent impulses.
INTRODUCTION A striking feature of the functional organization of the spinal cord is the way in which the excitatory and inhibitory effects of a stimulus tend to influence motoneurons reciprocally, causing contraction of certain muscles and inhibition of their antagonists. Thus, for example, Liddell and Sherrington (18) demonstrated in decerebrate preparations that the stretch of a muscle reflexly excited its own motoneurons, while inhibiting those of antagonistic muscles. Reciprocal innervation has been documented in spinal preparations as well ( 1). The reflexes in preparations which had recovered from asphyxiation of the spinal cord for about 3.5 min, in general did not exhibit the reciprocal principle (21). Especially during “late tone,” the stretch of flexor muscles elicited contractions in both flexors and extensors, as did nocuous stimuli. 1 This investigation Public Health Service.
was
supported
in
part
337 Copyright All rights
0 1976 by Academic Press, Inc. of reproduction in any form reserved.
by
Grant
NS-09493
from
the
U.
S.
338
TRUBATCH
AND
VAN
HARREVELD
Also the final phase of “secondary tone” was characterized by the absence of reciprocal innervation. In the present paper preparations and conditions are described in which, even without asphyxiation, spinal reflexes did not show a reciprocal distribution. METHODS Chronic and acute spinal cats were prepared respectively 2 to 3 weeks or 1 day before the terminal experiment. Under pentobarbital narcosis the dura was ligated at T 10-11, severing the spinal cord. In the terminal experiment the trachea of normal and spinal cats was cannulated under ether narcosis. The carotid arteries were ligated and one was cannulated for blood pressure recording. The preparation was decerebrated by cutting the brain ,stem, using the bony tentorium as a guide. Most of the forebrain was removed and the narcosis discontinued. The tendons of the quadriceps, the gastrocnemius-soleus, the semitendinosus, and the anterior tibia1 muscles on the homolateral and contralateral side were isolated and freed from the surrounding tissue. A strong cotton thread was attached to each of them. The condyles of the femurs and the distal end of the tibias were fixed on a vertical board with screws. The threads attached to the tendons were led over pulleys and loaded with 5-g weights to keep the threads under slight tension. Larger weights could be attached to the small weights to stimulate the stretch receptors in the muscles. Electromyograms were led off from the quadriceps and gastrocnemius-soleus muscles and in some experiments from the tibia1 muscle with electrodes placed in the muscle belly. The potentials, after amplification, were recorded on one or more channels of a Grass polygraph or a Physiograph, and in many experiments integrated by another channel. The blood pressure was monitored continually. RESULTS Preparations that had been operated the day before the terminal experiment showed an unexpected distribution of reflex activity. Table 1A shows four typical experiments. In all preparations the extensor muscles (quadriceps and gastrocnemius-soleus, q and g) were activated by stretch of their own tendons as well as of their synergist (gastrocnemius-soleus or quadriceps, g or q) . However, in contrast to the results expected from the principle of reciprocal innervation, stretching the antagonist flexors (semitendinosus and anterior tibial, s and t) of the homolateral leg also led in most instances to excitation of the extensors. Extensor activity was observed in three of the preparations (Table lA, 1, 2 and 3) after loading some of the muscles of the contralateral leg (quadriceps, semitendinosus,cq, CS) . Acute
Spinal
Preparations.
REFLEXFIGURES
339
Pinching of the foot, which elicited a marked contraction of the flexor muscles (flexion reflex), was expected to cause inhibition of the homolateral extensors. In most preparations, however, the extensor muscles contracted strongly on pinching the homolateral paw (Table 1A). Pinching the contralateral paw had in general no effect but caused in two preparations (Table lA, 3, 4) a slight contraction or inhibition of the extensors. Decerebrate Preparations. Decerebrate rigidity as evidenced by a pronounced extensor contraction, particularly in the front legs, appeared in most animals soon after the ether narcosis was discontinued. The hind legs, which usually showed less extensor rigidity, always exhibited brisk knee and ankle reflexes. In all preparations loading the tendons of the quadriceps, gastrocnemius, and tibia1 muscleswith weights of 200 or 500 g resulted in an enhancement of their electromyogram. These preparations often showed a significant reflex response only in the stretched muscle (Table lB, Fig. lA-J), although in some experiments contractions were elicited by synergists (Table IB, 7, 8). In preparations with a marked spontaneous background activity, or during activation of the extensor muscle by stretch, the contraction was often inhibited by simultaneous stretch of the flexors (Table lB, Fig. IK). However, in one preparation (Table lB, 7) stretch of the homolateral (and contralateral) semitendinocus muscles (s and csj caused contraction of extensors. TABLE
1 cs
A : Acute
1 2. 3 4
cg
ct
0 i 00 i
0 i0 0 0 i
P
CD
Spinal
Quad Gast Quad Gast Quad Gast Quad Gast
y’ 0” V) 0 0
0
+++++ A++ +++ +:+
0” 0 0 T -
B : Decerebrate i 7 8
Quad Quad Cast Quad Gast Quad Gast Tibia1
+++ +“+ +I+ 0
-
9
11
Quad Gast Quad Gast Quad Gast
+++ ++ O/$7
++
+++
33
+
+0+ ++ C : Chronic
10
0 0 0
++++++J+ ‘-;-“ +‘+’ +‘+ +‘+
i Spinal
+Y-
i!
++ +’
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a q-quadriceps. s-semitendinosus. g-gastrocnemius, t-anterior tibial, c-contralateral. p-pinching. B Strength oi contraction is roughly indicated by one to three + signs. inhibition by a - sign. effect by 0. Change of contraction to inhibition by stretch of the recording muscle is indicated notation +/ -.
++ :: ‘i++ and no by the
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FIG. 1. The response of the gastrocnemius-soleus (g) muscle to stretch of the quadriceps (q), semitendinosus (s), gastrocnemius-soleus (g) and tibia1 (t) muscles and to pinching the paw (p) of the homolateral leg. Stretch of the muscles of the contralateral side is indicated by cq, cs, cg and ct, and contralateral pinch by cp. The bottom record is the electromyogram and the top is its integral. Stretch or pinch was applied during the time indicated by the central line. All muscles were loaded with 500-g weights. Records A-M show that in this decerebrate preparation the gastrocnemius muscle responded only to loading of its own tendon. In records K-M, the gastrocnemius was first loaded with a 500-g weight. Subsequent loading of the homoiateral anterior tibia1 muscle caused a slight inhibition of the myogram of the stretched gastrocnemius muscle ; homolateral pinching inhibited the activity in this muscle completely, heterolateral pinching caused a slight increase. The animal’s spinal cord was then transected and records N-V show the altered response of this same muscle 2 hr later. The vertical scale indicates 150 mV for the myogram; the horizontal marker, in this and all other records, is 10 sec.
In all these preparations, pinching of the paw elicited a marked contraction of the homolateral flexor muscles as expected from the flexion reflex. In some of the experiments (Fig. lL, M) this caused inhibition of homolateral and excitation of the contralateral extensor muscles (crossed extension). However, in other experiments (Table lB, 7, 8) the nocuous stimulus elicited a contraction in the homolateral extensors. In some experiments this excitation was changed to inhibition when the muscle was previously activated by stretch (Table lB, 7, 8). Figures 2A and B show an example of such an experiment. In this case the electromyogram was recorded simultaneously from the quadriceps, gastrocnemius, and tibia1 muscles. Pinching the homolateral paw (Fig. 2A) led to a pronounced activation of the tibia1 muscle and simultaneously a small contraction of the antagonist extensors (quadriceps, gastrocnemius) . Pinching the contra-
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FIG. 2. Simultaneous recording of the electromyogram from the quadriceps, gastrocnemius, and tibia1 muscles. Records A and B are from a decerebrate preparation. In A the unloaded extensor muscles show a slight activation during both homolateral (p) and contralateral (cp) pinching of the paw. When the gastrocnemius was previously loaded with a 500-g weight (B), pinching resulted in a marked inhibition of the gastrocnemius myogram. This animal’s spinal cord was then transected and records C and D show similar responses 3 hours later. The vertical scale is 100 mV.
lateral foot caused activation of the quadriceps (as in crossed extension, but in addition there was a small effect on the tibia1 muscle. After the gastrocnemius was loaded with a 500-g weight (Fig. 2B), causing a marked increase of the myogram, pinching the homolateral foot caused a pronounced inhibition of the gastrocnemius activity and simultaneous tibia1 activation (as in reciprocal inhibiton). The reflex response of the decerebrate preparations to loading of muscles and to nocuous stimuli was in general in agreement with the tenet of reciprocal innervation, although exceptions were found. In several of the decerebrate preparations the spinal cord was transected subsequently and the experiment was repeated a few hours later. Figure lN-VL, which is a record of the same preparation as in Fig. lA-M in the decerebrate state, shows the effect of spinal transection. Loading the recording muscle (gastrocnemius, g) had now only a small effect but stretch of the flexor muscles (anterior tibia1 and semitendinosus, t and s) caused large extensor contractions. Also pinching the homolateral paw which caused in the decerebrate state complete inhibition of the extensor muscle now caused a marked contraction. In some instances it was possible to turn the contraction of extensors elicited by homolateral nocuous stimuli into inhibition by loading the extensor muscle. This was successful only when stretch of the extensor caused a marked excitation which was rarely observed in such preparations. Figure 2C. D is an example of such an observation. This preparation yielded Fig. 2A. B in the decerebrate state. Pinching the homolateral foot caused a simultaneous marked activation of both gastrocnemius and tibia1 muscles, and a smaller response of the quadriceps muscle (Fig. 2C). The response in the gastrocnemius was far more
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marked than it had been under the same circumstances in the decerebrate state. However, when the gastrocnemius was activated by stretch (Fig. ZD), homolateral pinch (p) produced flexor excitation and extensor inhibition as expected from the principle of reciprocal activation. Chronic Spinal Preparations. In chronic spinal preparations the reflex excitability increases with time and after 2 to 3 weeks the preparations may respond to a relatively minor stimulus applied to the skin with extensive movements of the hind legs of considerable duration. Table 1C and Fig. 3 show effects of muscle loading in some representative preparations. In addition to a considerable effect of loading the recording quadriceps or gastrocnemius muscles, the myograms of these muscles were also enhanced by loading of several of the other muscles both homolaterally and contralaterally. Sometimes the effect of loading an antagonist was greater than stretching the recording extensor muscle (Figs. 3A and B). Only rarely did stretch of a homolateral flexor muscle inhibit the spontaneous electromyogram of the unloaded recording extensor muscle (Table lC, 11). However, in several cases excitation of the recording quadriceps or gastrocnemius muscle by one of its antagonists was changed to inhibition when the recording muscle was previously loaded (Table 1C). In the experiment shown in Fig. 4A, B, stretch of the homolateral semitendinosus and anterior tibia1 muscles enhanced the electromyogram of the unloaded recording quadriceps muscle. After loading the recording muscle with a 500-g weight, stretch of the semitendinosus muscle caused a decrease of the electrical activity in the muscle (Fig. 4D). The enhanced activity when the load was removed from the muscle demonstrates that this is indeed due to inhibition and not to fading of the stretch reflex. Stimulation of the anterior tibia1 muscle had a biphasic effect, first enhancing, then depressing the electromyogram of the recording muscle (Fig. 4E). Again the enhanced activity after removal of the load on the anterior tibia1 muscle proved the latter to be due to inhibition. Homolateral and contralateral pinching of the paw led in general to an increase in the electrical activity of the unloaded extensor muscle (Table
FIG. 3. The responses of the quadriceps muscle of a chronic spinal preparation to stretch of four homolateral and four contralateral leg muscles, and to pinching of the paws. For an explanation of the records see Fig. 1. The muscles on the homolateral side were stimulated with 200-g weights; on the contralateral side 500 g was used. The vertical scale is 300 mV for the myogram.
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FIG. 4. The change of excitation to inhibition in a chronic spinal preparation. For an explanation of the records see Fig. 1. A, B, and C show the response of the gastrocnemius-soleus muscle to stretch of the semitendinosus and tibia1 muscles with 500-g weights and to homolateral pinching. In D, E, and F, a 500-g weight was attached to the recording gastrocnemius-soleus muscle a few seconds before stretch of the flexor muscles and of homolateral pinching. The vertical bar indicates 200 mV for the myogram.
1C and Fig. 31, J). The activation by homolateral pinching could in several experiments be changed to inhibition by loading the recording muscle. Figure 4C shows the enhancement of the electromyogram of the unloaded quadriceps muscle by pinching the homolateral paw; Fig. 4F shows the effect of the same stimulus on the loaded recording muscle which is a marked inhibition. DISCUSSION Stretch of a muscle elicits impulses in the primary (1A) and secondary (II) endings of the muscle spindles and in the 1B fibers from the Golgi tendon organs. Activation of the 1A system leads to excitation of the motoneurons of homonymous and synergist muscles and inhibition of antagonistic motoneurons. The tendon organs inhibit the motoneurons of the stretched muscle (autogenic inhibition) and reflexly excite antagonist motoneurons. A third group of thinner afferent fibers (group II), originating mainly in the secondary muscle spindle endings, usually cause a weak excitation of flexor motoneurons and inhibition of extensors (2, 3, 7, 17) although other effects have been observed (7. 11, 23 j. The widespread presynaptic inhibition of primary afferents by muscle, tendon, joint, and skin afferents further complicates this picture. In addition, the discharge of the motoneurons which innervate the intrafusal fibers of the muscle spindles is affected by stretch of the muscle (4, 10, 12, 20) and in this way indirectly modifies the discharge of the spindle afferents. A stimulus such as the stretch of a muscle therefore has excitatory as well as an inhibitory effect on its own motoneurons (9, 13, 19). Furthermore, such stimuli have effects not only on motoneurons of the stretched muscle, but on homolateral as well as contralateral synergists and antagonists. In all these reactions to stretch, except for the activation of the homonymous motoneurons by the IA fibers, interneurons are involved.
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Other peripheral receptors, in addition to the muscle stretch receptors, contribute to the input of the motoneurons. There are pressure receptors (Pacini’s corpuscles) and less well defined pain receptors in the muscle. The latter form a group of (thin) afferent fibers (group III) which elicit mainly flexion reflexes when activated. In the preparations used, larger and smaller cutaneous nerves were unavoidably cut. The injury to those nerves produces only a short-lasting discharge, after which they remain silent (22). In addition to this complicated peripheral innervation, the motoneurons are affected by descending activity from supraspinal centers. Especially the descending activity during decerebrate rigidity is of interest, Descending impulses excite the y motoneurons, causing the contraction of intrafusal fibers and an enhanced discharge of the primary sensory endings of the muscle spindles resulting in the reflex contraction of LYmuscle fibers, an activity known as the y loop (8, 15). Furthermore, stimulation of group II and III fibers elicits little or no reflex discharge of flexor motoneurons in decerebrate preparations, whereas after transection of the spinal cord impulses in these fibers cause large reflex responses, suggesting that during decerebrate rigidity descending impulses inhibit interneurons involved in these reflex activities (6, 16). The multiplicity of impulses impinging upon the motoneurons, both from the periphery and (in normal and decerebrate preparations) from higher centers determine the reflex figure in a given preparation under given circumstances. Reciprocal innervation is the reflex figure which was usually seen in decerebrate and spinal preparations. Other responses are possible, however. During voluntary movements both extensors and flexors are activated. The reflex figure can furthermore be changed markedly, not only by a change of the supraspinal input, as by spinal cord transection of a decerebrate preparation, but also by modification of the input from the periphery. Job (14) f ound that the enhancement of monosynaptic reflex responses during stretch of the gastrocnemius muscle in spinal preparations turned into a decrease when the muscle was cooled or its circulation impaired. Peripheral modification or reflex activity was also demonstrated by Clare and Landau (5), who showed that the response of the extensor digitorum brevis muscle to shocks applied to the tibia1 nerve was enhanced after crushing the innervation of the interosseus muscles. Also the present investigation provides an example of the modification of the reflex activity by changes in the afferent inflow. Stretch of a homolateral flexor muscle or homolateral nocuous stimuli caused in spinal preparations either contraction or inhibition of an extensor muscle depending on the state of retraction or stretch of the latter. When the tendon, which was detached from its insertion, was not loaded the muscle retracted, its length becoming even shorter than during maximal extension of the
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joints in the normal state. Under these circumstances the afferent outflow from the muscle will be at a minimum, which is borne out by the observation that in the decerebrate state the spontaneous electromyogram is small (Fig. 2A, B) or absent (Fig. IA-J). Such retracted extensor muscles tended to be activated in spinal preparations by homolateral flexor stretch and pinching of the paw. Loading the tendon of the recording extensor muscle with a 200- to 500-g weight causes an outflow of impulses in 1A fibers which activate the muscle reflexly and in 1B and group II fibers which both inhibit the extensor motoneurons. The result of homolateral flexor stretch and nocuous stimulation was then often an inhibition of the contraction caused by activation of the 1A fibers. The reversal of the reflex effect of homolateral flexor stretch and nocuous stimulation thus seems to be caused by a change in the afferent inflow from the recording extensor muscle. In the retracted state in which the afferent outflow from the unloaded extensor muscle is at a minimum, the excitatory components of homolateral flexor stretch and pinching of the paw apparently override the inhibitory effect of these stimuli, causing the extensor contractions elicited consistently by these stimuli in acute and chronic spinal preparations (Table 1A and C). In the loaded state of the recording extensor muscle the combined inhibitory effect of 1B and group II stimulation, with the inhibitory component of the reflex activity caused by homolateral flexor stretch and pinch of the paw, seems to be able to overcome the excitatory component of the latter stimuli and the effect of 1A stimulation. In decerebrate preparations the extensor muscle usually exhibited reciprocal innervation. Even in the retracted state, homolateral flexor stretch or nocuous stimulation caused extensor contraction only occasionally (Table lB, Fig. 2A, B) The supraspinal inflow in these preparations which has excitatory as well as inhibitory components (see above) apparently tends to tip the balance of excitatory and inhibitory impulses impinging on the extensor motor neurons usually to the inhibitory side. The conclusion can be drawn from these experiments that reciprocal innervation does not depend on fixed neural connections in the spinal cord but is the result of the ratio of excitatory and inhibitory impulses reaching the motoneurons. A ratio favoring reciprocal innervation seems usually to have been present under the conditions of the experiments on reflex figures reported in the past. REFERENCES 1. BALLIF, L., J. F. FULTOK, decerebrate knee-jerks, break-shocks. Proc. R. 2. BIANCONI, R., R. GRANT,
and E. G. I. LIDDELL. 192.5. Observations on spinal and with special reference to their inhibition by single Sot. Series B, 98: 589-607. and D. J. REIS. 1964. The effects of extensor muscle
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spindles and tendon organs on homonymous motoneurones in relation to -y-bias and curarization. Acta. Physiol. Stand. 61 : 331-347. BIANCONI, R., R. GRANIT, and D. J. REIS. 1964. The effects of flexor muscle spindles and tendon organs of homonymous motoneurones in relation to y-bias and curarization. Acta. Physiol. Stand. 61: 348-356. BROWN, M. C., D. G. LAWRENCE, and P. B. C. MATTHEWS. 1968. Reflex inhibition by Ia afferent input of spontaneously discharging motoneurones in the decerebrate cat. 1. Physiol. (London) 198: S-7P. CLARE, M. H., and W. M. LANDAU. 1975. Distal hind-limb reflex responses in cats with largely intact spinal reflex circuits. Exp. Newel. 46: 47C-495. ECCLES, R. M., and A. LUNDBERG. 1959. Supraspinal control of interneurones mediating spinal reflexes. J. Physiol. (London) 147: 565-584. ECCLES, R. M., and A. LUNDBERG. 1959. Synaptic actions in motoneurones by afferents which may evoke the flexion reflex. Arch. Ital. Biol. 97 : 199-221. GRANIT, R., and B. R. KAADA. 1952. Influence of stimulation of central nervous structures on muscle spindles in cat. Acta. Physiol. Stand. 27: 130-160. GRANIT, R., J.-O. KELLERTH, and T. D. WILLIAMS. 1964. Intracellular aspects of stimulating motoneurones by muscle stretch. J. Physiol. (London) 174 : 435-452. GRILLNER, S. 1969. Supraspinal and segmental control of static and dynamic ymotoneurones in the cat. Acta. Physiol. Stand. 76: suppl. 327. HOLMQVIST, B., and A. LUNDBERG. 1961. Differential supraspinal control of synaptic actions evoked by volleys in the flexion reflex afferents in alpha motoneurones. Acta. Phq’sioE. Stand. 54: suppl. 186. HUNT, C. C. 1951. The reflex activity of mammalian small-nerve fibers. I. Physiol. (London) 115 : 456-469. HUNT, C. C. 1952. The effect of stretch receptors from muscle on the discharge of motoneurones. I. Physiol. (London) 117 : 359-379. JOB, C. 1953. Uber autogene Inhibition und Reflexumkehr bei spinalisierten und decerebrierten Katzen. Pfliigers Arch. Ges. Physiol. 25’6: 406-418. KOBAYASHI, Y., K. OSHIMA, and I. TASAKI. 1952. Analysis of afferent and efferent systems in the muscle nerve of the toad and cat. J. Physiol. (London) 117: 152-171. KUNO, M., and E. R. PERL. 1960. Alteration of spinal reflexes by interaction with suprasegmental and dorsal root activity. J. Physiol. (London) 151: 103-122. LAPORTE, Y., and P. BESSOU. 1959. Modification d’excitabilitb de motoneurones homonymes provoqueCs par l’activation des fibres affCrentes d’origine musculaire du groupe II. J. Physiol. (Paris) 51: 897-908. LIDDELL, E. G. T., and C. SHERRINGTON. 1925. Further observations on myotatic reflexes. Proc. R. Sot. Series B, 97 : 267-283. MCINTYRE, A. K. Central actions of impulses in muscle afferent fibers. In “Muscle Receptors,” Handbook of Sensory Physiology. Vol. III/Z, C. C. Hunt [Ed.], pp. 235-288. PROSKE, U., and D. M. LEWIS. 1972. The effects of muscle stretch and vibration on fusimotor activity in the lightly anesthetized cat. Brain Res. 46: 5549. VAN HARREYELD, A., and J. TRUBATCH. 1974. Reflex figures during asphyxial rigidity. Exp. Nmrol, 45 : 161-173. WALL, P. D., S. WAXMAN, and A. I. BASBAUM. 1974. Ongoing activity in peripheral nerve : Injury discharge. Exp. Nealrol. 45 : 576-589. WILSON, V. J., and M. KATO. 1965. Excitation of extensor motoneurons by group II afferent fibers in ipsilateral muscle nerves. J. Newophysiol. 28: 545-554.
